Various wireless communication systems in accordance with a Third Generation Partnership Project (3GPP) (registered trademark, the same hereinafter) standard are known.
In wireless communication systems in accordance with a Long Term Evolution (LTE) (registered trademark, the same hereinafter) standard defined by the 3GPP, a logical communication path (a user plane path) which is used in transmission of user data is established between a packet gateway (PDN gateway) and a user apparatus via a serving gateway and an evolved Node B (eNB), which is a cellular base station. Accordingly, the user apparatus can perform communication with an external network (such as the Internet) using the established user plane path.
A frequency band which is used in a wireless communication system in accordance with the LTE standard requires a license for transmission of radio signals. Hereinafter, a frequency band requiring a license for transmission of radio signals is referred to as a licensed band.
Wireless communication systems in accordance with a standard other than the LTE standard include a wireless communication system which performs wireless communication based on a random access scheme in a frequency band requiring no license for transmission of radio signals. Hereinafter, a frequency band requiring no license for transmission of radio signals is referred to as a shared frequency band.
Since a plurality of wireless communication systems and a plurality of wireless terminals of a plurality of owners are mixed in a shared frequency band, wireless communication in the shared frequency band may be more unstable in terms of communication quality than wireless communication in a licensed band. However, wireless communication systems in shared frequency bands, such as a wireless LAN system defined by IEEE 802.11, are improved in terms of throughput every year, and are spreading as one of major wireless accesses. In particular, the spread of smartphones remarkably increases opportunities to use wireless LAN systems. Moreover, in a shared frequency band, a license is not required for transmission of radio signals, and thus various wireless terminals have spread.
Shared frequency bands, which are frequency bands available to wireless LAN systems, are assigned to 2.4 GHz, 5 GHz, and 60 GHz. In 2.4 GHz and 5 GHz, which are microwave bands, a bandwidth which can be used by wireless LAN systems in Japan is about 500 MHz. A comparable bandwidth or more is also assigned in countries other than Japan as a band available to wireless LAN systems.
In order to cope with a rapid increase in traffic in a licensed band, it is very important to increase communication capacity of a wireless communication system by using the shared frequency bands. A base station apparatus that performs wireless communication in a shared frequency band is expected to cope with a broad coverage or a high throughput by performing advanced processing, such as a directional antenna, massive MIMO, OFDMA, and analog beam forming. Moreover, improvement in throughput of uplinks and downlinks by multi-user MIMO has also been studied.